javascript.void(0)
The void operator evaluates the given expression and then returns undefined.
The void operator is often used merely to obtain the undefined primitive value,
usually using “void(0)” (which is equivalent to “void 0”). In these cases,
the global variable undefined can be used instead (assuming it has not been
assigned to a non-default value).
An explanation is provided here: void operator.
The reason you’d want to do this with the href of a link is that normally,
a javascript: URL will redirect the browser to a plain text version of the
result of evaluating that JavaScript. But if the result is undefined,
then the browser stays on the same page. void(0) is just a short and
simple script that evaluates to undefined.